1. Field of the Invention
The present invention relates to interference type photocathodes having coatings to selectively reject radiation at one frequency and selectively detect radiation at a nearby wavelength.
2. Description of Related Art
U.S. Pat. No. 4,614,871 to Driscoll discloses a photodiode constructed of a metal (e.g. nickel) that emits electrons only in response to far-UV radiation of 140 nanometers shorter wavelengths. A window (e.g. magnesium fluoride) is positioned at the entrance to the photodiode to filter out far-UV radiation with wavelengths shorter than 100 nanometers. Thus, the photodiode is sensitive to radiation in a wavelength range no greater than 100 to 140 nanometers, and its sensitivity to radiation outside that range is less than 10% of its maximum sensitivity within the range.
U.S. Pat. No. 3,638,059 to Taylor discloses a photometer that comprises two basic components: a window through which radiation passes, and a cathode which has a photoelectric surface that is excited by desired radiation that passes through the window. The window used in the Taylor photometer blocks out penetration of all visible and near ultraviolet, whereas the infrared and lower frequency radiation which is transmitted does not effect the cathode. Therefore, the structure is suitable for detecting radiation between 150 and 800 angstroms.
U.S. Pat. No. 4,680,504 to Helvy et al. discloses an electron discharge device with a photoemissive cathode which is disposed within an envelope for providing photoelectrons in response to radiation incident thereon. The device is improved by forming the face plate from an optical filter which transmits radiation predominantly in a first portion of the electromagnetic spectrum. Furthermore, the photoemissive cathode has an intrinsic responsivity extending from said first through a second portion of the spectrum. However, the combination of the filter face plate and the photoemissive cathode limits the tube to a responsivity within a spectral range of s id first portion of the electromagnetic spectrum
U.S. Pat. No. 4,698,496 to Dolizy discloses a photoelectric detection device comprised of a vacuum chamber provided with a window having a substrate which bears a photocathode on the internal surface of the vacuum chamber. The device is sensitive to incident luminous radiation between a short wavelength bottom threshold .lambda..sub.1 and a longer wavelength upper threshold .lambda..sub.2. Variations in photoelectric power are connected with the composition and thickness of the layers, the probability of electron emission, and the topology of the surfaces, these parameters having a different effect according to the wavelength range of the incident beam. The invention of Dolizy suppresses the influence of the sensitivity threshold .lambda..sub.2 of the detection device by filtering the incident light spectrum and suppressing this high threshold .lambda..sub.2. This filter can be an interference filter constructed by a series of layers of material with high and low optical indices. For this purpose, a low pass filter (one which passes shorter wavelengths) is created which cuts off the longer wavelength for which the transmission of the low pass filter is perhaps in the region of 10%. Light passes through the interference filter producing at the outlet a light beam of which the wavelengths are limited in the upper part and possibly in the lower part according to characteristics of the filter(s) in the context of the invention. This beam of filtered wavelengths is absorbed in the photocathode to generate electrons emitted over the complete surface of the photocathode.
D. Kossel et al, Physics of Thin Films, edited by G. Hass and R. E. Thun, Academic Press, New York, 1969, describe coatings designed to absorb radiation at a particular wavelength.